In mechanical dishwashing generally two cleaning cycles, usually separated by intermediate rinsing cycles with pure water are used. In the two cleaning cycles, different products are utilized. In the first or true cleaning cycle, alkaline-reacting agents are employed for the loosening and emulsifying of the food residues. In the after-rinsing or clear-rinsing bath, on the other hand, special clear-rinsing agents are employed. The latter should possess a good wetting power and be able to reduce the surface tension of the after-rinsing water to such a degree that it drains in a film-like manner from the dishes and leaves no visible deposits, such as lime spots or other impurities.
Because of the violent agitation of the liquor in the dishwasher, these clear-rinsing agents have to be as low-foaming as possible. The customary anionic wetting agents, however, such as higher-molecular-weight alkyl sulfates or alkyl sulfonates or aralkyl sulfonates are not generally usable because they foam too much. In practice, therefore, mostly nonionic tensides based on ethylene-oxide adducts to fatty alcohols, alkylphenols, or polypropylene glycols of higher molecular weights are employed. These products, however, were also found in actual practice to be not sufficiently low-foaming in the concentration range, required for a sufficient wetting effect.
These adducts have been found to cause disturbances due to excessive foam formation particularly in commercial dishwashing machines which have a very high rate of water circulation and a very high return rate of the clear-rinsing liquor into the main rinsing cycle. The same difficulties may also arise in home dishwashing machines. Even with the use of relatively low-foaming ethylene-oxide adducts, it is therefore necessary to add anti-foaming agents to the clear-rinsing agents. Suitable nonionic alkoxylation products are those which are slightly soluble at rinsing temperatures, such as ethylene-oxide adducts to higher alcohols or alkyl phenols with a low degree of ethoxylation or suitable adducts of ethylene oxide and propylene oxide. Such products possess, however, no wetting effect at the application temperatures and thus present a burden for the clear-rinsing agent.
In addition to this, the applied wetting agents should possess a good biological degradability and a low toxicity toward the organisms living in water. These features were not previously found to be available along with a good wetting and draining effect.
In U.S. Pat. No. 3,775,330, a low-foaming clear-rinsing composition adapted for mechanical dishwashers was disclosed, consisting essentially of (A) an adduct of from 5 to 20 mols of ethylene oxide and from 1 to 10 mols of propylene oxide to an alkanediol having a linear alkane chain with from 10 to 20 carbon atoms and vicinal, non-terminal hydroxyls, and (B) a lower organic carboxylic acid having from 2 to 6 carbon atoms and sequestering properties toward salts causing water hardness, wherein the weight ratio between component (A) and component (B) is from 1:0.2 to 1.3; together with the process for the mechanical washing of dishes comprising subjecting dirty dishes to the action of a washing solution, subjecting the washed dishes to at least one clear-rinising solution and recovering said washed dishes, the improvement consisting of utilizing an aqueous solution containing from 0.01 to 1.0 gm. per liter of an adduct of from 5 to 20 mols of ethylene oxide and from 1 to 10 mols of propylene oxide to an alkanediol having a linear alkane chain with from 10 to 20 carbon atoms and vicinal, non-terminal hydroxyls, as said at least one clear-rinsing solution.
In U.S. Pat. No. 3,779,934, a clear-rinsing composition of an aqueous solution of an adduct of from 3 to 30 mols of ethylene oxide to alkanediols with a linear alkane chain of from 10 to 20 carbon atoms and having vicinal non-terminal hydroxyls, was disclosed and employed.
In U.S. Pat. No. 3,969,134, a clear-rinsing composition of an aqueous solution of (a) from 50% to 100% by weight of an adduct of from 5 to 20 mols of ethylene oxide and from 1 to 10 mols of propylene oxide to a secondary alkanol having a linear alkane chain of from 10 to 20 carbon atoms and (b) from 0 to 50% by weight of an adduct of from 5 to 10 mols of ethylene oxide to a secondary alkanol having a chain length of from 11 to 15 carbon atoms was disclosed and employed.